improve quicklist insert head/tail while head/tail node is full. (#9113)
improve quicklist insert head/tail while head/tail node is full. (#9113)

In _quicklistInsert when `at_head` / `at_tail` is true, but `prev` / `next` is NULL,
the code was reaching the last if-else block at the bottom of the function,
and would have unnecessarily executed _quicklistSplitNode, instead of just creating a new node.
This was because the penultimate if-else was checking `node->next && full_next`.
but in fact it was unnecessary to check if `node->next` exists, if we're gonna create one anyway,
we only care that it's not full, or doesn't exist, so the condition could have been changed to `!node->next || full_next`.

Instead, this PR makes a small refactory to negate `full_next` to a more meaningful variable
`avail_next` that indicates that the next node is available for pushing additional elements or
not (this would be true only if it exists and it is non-full)
JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis